The Pine Processionary Caterpillar can pose a major risk to children and adults, causing dermatitis, eye damage and severe allergic reactions.
In pets, particularly dogs, they can cause breathing difficulties, vomiting and foaming at the mouth.
Pets showing such symptoms should be taken to the vet immediately.
The caterpillars, which come out in force in the spring, are covered with tiny barbed hairs that contain a protein called thaumetopoein.
Dogs are most at risk as they can pick up the hairs in their paws which they then lick when they become irritated, causing the poison from the hairs to spread to the mouth.
Amputations of the tongue or nose can be necessary to prevent the poison reaching the throat and causing suffocation.
Pine forests or areas with pine trees are the most at risk locations.
Councils may spray down municipal parks but they cannot always be relied upon.